This proposal, in response to RFA-AI-02-002, seeks funding to study long-lasting immunity (e.g. immunological memory or recall) to smallpox (variola) in individuals who were previously vaccinated in the United States commercially approved smallpox vaccine (preparation of live Vaccinia virus). In the aftermath of September 11, 2001, it is now critical to better define mechanisms of smallpox protective immunity or disease resistance in the general population and in those who may be classified immune compromised and therefore, considered at a higher risk of infection. Though smallpox was considered a disease of historical interest only, since its earlier eradication, it as been known to be a significant pathogen for potential bioterrorist activities. Investigation of long-lasting immunity using current state-of-the-art techniques or methodologies may help determine which individuals are better prepared to serve as "first-line" responders. In addition, this information may help determine a better strategy for use of limited, non-universally administrable vaccine material available today. Specific Aim 1: To assess smallpox-specific long-lasting immunologic responses and correlate to donor age and time since last vaccination in healthy individuals that have previously received a smallpox vaccination. Hypothesis: Initial childhood vaccine administration will be associated with differing levels of smallpox-specific host immune responses that will vary with number of immunizations given and the time since last vaccination. Specific Aim 2: To assess smallpox-specific long-lasting immunologic responses in HIV- and HIV+ individuals through collaboration with the NIAID DAIDS ACTG and correlate to age and infection status in individuals previously having smallpox vaccination. Hypothesis: HIV infection, CD4 or viral load status, time since last vaccination and host age will each be associated differing levels of smallpox-specific host immune responses. Specific Aim 3: To viral epitope map and evaluate the CD4/CD8 immune subset critical memory component of virus-specific cell-mediated responses in healthy and immune comprised study participants. Herd immunity or natural boosting is absent from this model, it represents a unique opportunity to extensively study pathogen specific immunological memory. Hypothesis: Response level and/or frequency of CD4 or CD8 cells to various immunodominant viral epitopes will change with age, time since last vaccination or presence of disease (e.g, HIV).